Telephone system



p 8, 93 c. P. STOCK-ER 1,974,0 7;-

TELEPHONE sY.STEM

Filed.April 21. 193a s Sheets-Sheet 1 l m 1%2-"l a 3,). &

N s 1 n 1 l E w- INVENTOR u c./? STOCKER A T TORNEV c. P STOCKER1,974,087

TELEPHONE SYSTEM Sept. 18, 1934.-

FiledApril 21., 1955 5 Sheets-Sheet 2 f &

INVENTOR 61/? STOCKER BY A ORN Y Sept. 18, 1934-. I Q p STQQKER 71,974,087

TELEPHONE SYSTEM Filed.April 21 1933 3 Sheets-Sheet 5 INVENTOR By c. P.STOCKER 772.77

A TTORNEY Patented Sept. 18, 1934 UNETED STATES PATENT GFFlfiE TELEPHONESYSTEM Application April 21, 1933, Serial No. 667,211

17 Claims.

This invention relates to telephone systems and more particularly tosystems which include one or more unattended automatic oflices.

The object of the invention is to provide im- 5 proved arrangements forthe supervision of unattended automatic telephone ofiices.

In telephone systems which include an unattended automatic ofiice it isusual to provide means for transmitting trouble alarms from theunattended office to an attended automatic or manual ofiice. Systemsproviding for the transmission of trouble alarms from a branch ofiice toa main ofiice are disclosed in Patent 'No. 1,196,876, granted to F.Lubberger, September 5, 1916, in Patent No. 1,616,738, granted to W. A.Benson, February 8, 192'? and in Patent No. 1,805,726, granted to N. H.Saunders, May 19, 1931. The supervisory oifice may be a manual oificewith ring-down trunks incoming from at; the unattended automatic office,in which case the usual direct current or ringing current alarmtransmitting circuits are ineffective.

According to this invention means is provided at an unattended automaticofiice for connecting a high potential source of current to bothconductors of one of the ring-down trunks leading to a main ofiicecomprising a magneto switchboard. At the magneto end of the trunk, thereis provided alarm-initiating apparatus which responds to the connectionof the alarm voltage 'Without responding to normal ringing voltage.

A feature of the invention is the connection of each conductor of anincoming trunk through a space cischarge device to the winding of analarm relay, whereby the connection of a source of high potential toboth of said conductors at their distant end is effective to operate therelays and initiate an alarm.

Another feature of the invention is the connection of two spacedischarge lamps in series across the conductors of anincoming trunk, themid-point of said bridge being connected through a retard coil and thewinding of an alarm relay to ground, whereby the relay is operativelyresponsive to the breakdown of the lamps when a high potential isconnected to the distant end of both of said trunk conductors andoperativelynnaifected by the breakdown of the lamps when a highpotential is induced or bridged across the distant ends of said'conductors.

The drawings, which form a part of this specification, illustratescheinatically'a portion of a 55 telephone system arranged in accordancewith the features of this invention. It is, of course, understood thatthe invention is not limited in its application to the particular systemillustrated. Referring to the drawings,

Fig. 1 shows, in diagrammatic form, a subl scribers station A and line10, a line-finder LF having access to the line 10, a selector switch S1,and two two-way trunk circuits. T1 and T2 all located in an unattendedautomatic branch office; 1

Fig. 2 shows, in diagrammatic form, a toll intermediate selector TISassociated with trunk circuit T2, a connector C accessible to theselector TIS, a subscribers line 99 and station B, a miscellaneous alarmcircuit AL, and emergency alarm apparatus EA.1 associated with the trunkcircuit T2 for use in transmitting a trouble-alarm signal to a distantmain oifice, all of which equipment is located in the branch office; and

Fig. 3 shows, in a diagrammatic form, two two-way trunk circuits RDl andRD2 in a manual main ofiice associated with trunks 140 and 150 extendingbetween the main oiiice and the unattended branch ofiice of Figs. 1 and2; Fig. 3 also shows emergency alarm apparatus EA2 associated with thetrunk 150, a cord circuit CC, operators position equipment OP, and adialing cord DC.

The trunk circuit RD2 terminating the trunk 150 at the main ofiice andthe trunk circuit T2 terminating this trunk at the branch office areshown in detail while the trunk circuits EDI and T1 terminating thetrunk 140 are represented by the enclosures within which thesedesignations are located. The trunk circuits T1 and R111 are the same asthe trunk circuits T2 and RDZ, respectively, except that there is noemergency alarm apparatus provided for trunk 140. The selector andconnector. switches are 9 each represented. schematically by its set ofbrushes and one set of terminals, the associated apparatus and circuitconnections being omitted. These switches may be of the'well known,twomotion step-by-step type, for a description of 100 the structure andoperation of which reference may be had to pages 53 to 65, inclusive, ofthe 2nd edition of Automatic Telephony by Smith and CampbelLpublished in1921. The toll intermediate selector switch T18 is provided with 105 afourth conductor over which supervisory signals are transmitted from theconnector C to the trunk T2 for'controlling the cutting-in of therepeating coil of trunk T2. Reference may be had to the patent to- B. G.Dunham No. n

1,849,662, granted March 15, 1932, for a disclosure of a tolltransmission selector and atoll connector having the samesupervisory-control features as the trunk T2 and connector C.

By placing Figs. 1, 2 and 3 adjacent each other in the order named, thedrawings represent a system which includes an unattended automaticbranch ofiice and a main ofiice equipped with a magneto switchboard. Thetrunks 140 and 150 represent a group of two-wire two-way trunksconnecting the main and the branch of-' fice. At the branch office thesetrunks are connected to terminals in the banks of selector switches forseizure on calls outgoing to the main office and are also connected toincoming selectors for use on calls incoming from the main omce. Theassociated trunk circuits in the branch ofiice are arranged to transmitan impulse of ringing current to the main office upon seizure by any ofthe selectors to whose terminal banks the trunks are connected. At themain office the trunks terminate at an operators position, ring-downsignals being provided for responding to ringing current received fromthe. branch ofiice. Each trunk has two 'jacks associated therewith, onefor use .in esnals to the main ofiice whenever conditions arise whichrequire the attention of a maintenance attendant. To this end, themiscellaneous alarm equipment AL in the branch ofiice is provided with asingle motion step-by-step trunkselectingswitch TS for selecting when atrouble condition arises, any one of the idle outgoing trunks. When atrunk is thus seized, a trouble indicating tone of the desired characteris transmitted to the main office; One of the outgoing trunks is alsoprovided with emergency alarm apparatus for use in indicating powerfailure or other emergency arising at the branch ofiice. In the drawingssuch alarm apparatus is shown in Fig. 2 associated with the trunk 150and the trunk circuit T2 of Fig. 1. The trunk 140 represents theremaining trunks of the group, none of which are arranged for use ingiving emergency alarm signals. When an emergency arises, the trunk 150is appropriated for transmission of the alarm signal irrespective of itsidle or ,busy condition, means being provided for cutting off any lineto which the trunk may be connected when seized.

The operation of the trunk circuits T2 and RD2' on regular calls will befirst described. .011 a call from the main oifice to the branch omce,the operator inserts the calling plug 181 of the cord circuit CC intothe talking jack 163 thereby closing a circuit for operating relay 164;Relay 164 disconnects'signal relays 155 and 156 from the conductors oftrunk 150, bridges the impedance coil 160 across these conductors, andestablishes a talking connection from. the trunk through condensers 161and 162 to the conductors of jack 163. The bridging of impedance coil160 across conductors 151 and 152 causes the operation of the incomingline relay 3'7 of trunk circuit T2 in the branch oflice. The circuitfor, operating relay 37 may be traced from battery through-the upperwinding of relay 37, resistance 45, inner lower back contact of relay47, lower back contact of relay 33, conductor 52, lower normally closedfront contact of relay 5'7, over conductor 152 of trunk 150, throughimpedance coil 169 of trunk circuit RD2 in the main office, through theinner lower back contact of relay 166, outer front contact of relay 164,conductor 151 of trunk 150, upper normally closed front contact of relay5'7, conductor 51, upper back contact of relay 33, inner upper backcontact of relay 4'7, resistance 46, and through the lower winding ofrelay 37 to ground. Relay 37 closes a circuit for operating relay 24,this circuit being traced from battery, through the winding of relay 24,inner upper back contact of relay 22, lower front contact of relay 3'7to ground at the back contact of relay 36, Relay 37 also closes a bridgeacross conductors 16 and 17 leading to the incoming selector TIS tooperate the line relay (not shown) of the selector; this bridge istraced from conductor 16 through the uppermost back contact of relay 22,left upper winding of repeating coil 46, uppermost back contact of relay39, resistance 42, left lower winding of repeating coil 40, inner upperback contact of relay 38, upper front contact of relay 37, middle upperback contact of relay 22, to conductor .17. Relay 24 closes a circuitfor operating relay 31, connects ground to the sleeve conductor 18leading to the toll intermediate selector T15, and connects ground overconductor 26 to the sleeve terminal of trunk T2 in the banks of allselectors having access thereto as a busy indicating condition. Theoperation of relay 31 disconnects ground from the conductor leading tothe all-trunks-busy register 29. The ground connected to conductor 18serves as a holding ground for the switches or" the toll train and alsoserves to operate relay 66 of the emergency alarm apparatus EA1. Thefunction-of relay 66 will be hereinafter de scribed in connection withthe operation of the apparatus EA-l. 1

When the operator in the main office inserts the plug 191 of a dialingcord DC into jack 165, a circuit is closed for operating relay 166.Relay 166 bridges the interrupter contacts of dial 190 across conductors151 and 152 in place or" the impedance coil 160. When the operator dialsthe number of the called station, the incoming line relay .37 of trunkcircuit T2 in the branch oflice is alternately released and reoperatedin response to each dial impulse. The impulses are repeated by the uppercontacts of relay 37 over conductors 16 and 17 to selectively operatethe switch TIS and the succeeding switches employed inestablishingconnection with the called line in the well known manner.Upon the release of relay 37 in response to the first impulse of eachdigit, relay 25 operates and, being slow in releasing, remains operateduntil all of the impulses in a train have been received. The operationof relay 25 prevents premature operation of relay 47. In response to theimpulses repeated over conductors 16 and 17, the selector T18 andconnector C are selectively operated to extend the connection to thecalled line, which is represented by line 99 and station B. When dialingis completed the plug 191 of cord DC is removed from jack 165, releasingrelay 166. If the called line is idle, ground (not shown) at theconnector C is connected over the control conductor and through thelowermost brush of selector TIS to the. control conductor 19, therebycausing the ing of relay 47, to battery. Relay 47 looks through itslowermost front contact over conductor 26 to ground at the front contactof relay 24. A short circuit, normally existing around the lower windingof relay 39, and the operating path for relay 47 are opened after thelocking path is closed. With relay 47 operated, the right-hand windingsof repeating coil 40 are inserted in series with the windings of relay3'7, and the condenser 44 is bridged across the inner ends of thesewindings. Signaling current is transmitted from connector C over line 99in the well known manner, and when the called subscriber answers, theground connected to the control lead of connector C is replaced by abattery potential (not shown) thereby causing the operation of relay 39,the circuit connection from conductor 19 being traced through the rightback contact of relay 25, the lower winding of relay 39, the lowermostfront contact of relay 47, over conductor 26 to ground at the frontcontact of relay 24. Relay 39 locks through its upper winding and innerfront contact over conductor 26 to the ground at relay 24. With relay 39operated, the impulse bridge across conductors 16 and 17 is opened; andthe windings of relay 41 are connected through the left windings ofrepeating coil 40 to these conductors to supply the called station withtalking current. Relay 41 connectsground to conductor 23 but theconnection is under the control of the operator in the main orlice sincethe release'of relay 37 opens the circuit for relay 41. Relay 24supplies the ground over sleeve conductor 18 for holding the selector HSand connector C. The talking currents are inductively transmittedthrough the windings of the repeating coil '40 in the trunk circuit T2and through condensers 161 and 162 of trunk circuit RD2. The potentialnormally applied to conductors 151 and 152 being insufficient to breakdown the resistance of lamps 171 and 172, the alarm circuit EA-2 doesnot interfere with the talking circuit. When the calling plug 181 isremoved from jack 163, upon the termination of the call, relay 164releases opening the bridge across'the trunk 150, thus causing therelease of relay 37 of trunk circuit T2. Relay 37 in turn causes therelease of relays 41, 24 and 31. The release of relay 24 also removesground from conductor 18, thereby causing the release of relay 66 andthe release of the intermediate toll selector TIS and connector C, theseswitches being restored to normal in the usual manner.

Assuming the subscriber at station A to have originated a call, that theline-finder LF has extended line 10 to the selector S1, and that thebrushes of this selector have been selectively advanced in response tothe dialing of a digit by the calling subscriber, and that the trunkcircuit T2 and trunk 150 have been seized, the line relay 21 of trunkcircuit T2 is operated in a loop circuit over the line 10. Relay 21closes obvious circuits for operating relays 22 and 24. 24 connects aguarding ground over conductor 26 to the sleeve terminals of this trunkto prevent its seizure by another selector and over conductor 13 to thesleeve terminal of the trunk hunting switch TS of the alarm circuit AL.Relay 22 looks to ground at the inner lower front contact of relay 24,disconnects ground from the conductor leading to the all-trunks-busyregister 29, closes a circuit for operating relay 31, and connects thesource of ringing current 2'7 through the lowermost front contact ofrelay 22,

Relay condenser 28, the lower back contact of relay 38, and over thering conductor of selector S1 to transmit an audible ringing signal tothe calling station. The ground connected through the inner lower frontcontact of relay 24, over conductor 18, is effective to operate relay 66of the emergency alarm apparatus EA-1. With relays 22 and 24 operated, acircuit is closed for operating relay 25; and with relays 22 and 31operated, a circuit is closed for operating relay 32. Relay 32 closes acircuit for operating relay 33. Relay 33 closes circuits for operatingrelays 35 and 36, connects the grounded side of the ringing currentsource 34 to conductor 51 and closes a signaling circuit from the sourceof ringing current .34, through the outer lower front contact of relay33, over conductor 52, lower normally closed front contact of relay 57,over conductor 152 of trunk 150, outer lower back contact of relay 166,condenser 162, outer lower back contact of relay 164, through thewinding of relay 156 to ground. Relay 156 responds to alternatingcurrent, thereby extending the ringing circuit through its frontcontact, the winding of drop signal 158, and resistance 159 to ground.The winding of signal 158 is then energized in parallel with the windingof relay 156 and the operation of drop signal 158. closes a circuit tothe supervisory (night alarm) signal 180. The operation of relay 35causes the release of relay 31, and closes a circuit for operatingrelay'47. The release of relay 31 causes the release of relays 32 and 33in turn. The release of relay 33 causes the release of relay 36 andopens the signaling circuit from source 34. Relay 35 remains operated,being locked through its lower front contact to ground at relay 22.Relay 36 is slow in releasing to prevent the premature op eration ofrelay 38 in case relay 37 responds to potential surges over the trunkconductor when relay .33 releases. Relay 47 inserts the righthandwindings of repeating coil 40 in series with the windings of relay 3'7and the conductors of trunk 150, and connects condenser 44 across theinner ends of these repeating coil windings.

When the operator at the magneto oflice answers the call by insertingthe answering plug of an idle cord such as CC in the jack 163, relay 164operates closing the talking circuit through condensers 161 and162 andbridging impedance 160 across the conductors of trunk 156 to cause theoperation of relay37 of trunk circuit T2 as hereinbefore' described.With relays 22 and 37 both operated and relay 36 released relay 38operates. Relay 38 locks under control of relays 22 and 24 and closes acircuit for energizing the upper winding of relay 39. Relay 39 operatesand locks through its inner upper front contact, over conductor 26, toground at relay 24. Relay 38 also opens the audible ringing tonecircuit, and transfers the calling line loop from the windings of relay21,-through the left-hand 'windings of repeating coil 40, to thewindings of relay 41. Relay 21 releases and relay 41 operates; relay 41connects ground to conductor 23 thus holding relay 24 operated.

When the calling subscriber replaces the receiver on the receiver hook,relays 41 and 24 release in turn. The release of relay 24 causes 7 therelease of relays 39, 4'7 and 25. Relay 25 is slow in releasing toprovide a momentary disconnection of the holding ground from sleeveconductor 26, thus enabling the return of switches S1 and LE to normalin the usual manner. If the operatorat the main oifice has not I as:

yet removed the answering plug from jack 163, relays 3'7, .38, 22 and'35are .held operated, the release of relay25connecting a guarding groundpotential from. the inner lower front contact of relay 38, through-theleft back contact of relay 25 over conductor 26 to the sleeve terminalof trunk circuit T2 in the banks of the first selectors and also over.conductor 13 to the sleeve terminal of this trunk circuit in the bank ofswitch TS of the miscellaneous.alarm'circuitAL. No disconnect'signal isgiven to the main office operator.

.When the operator disconnects, relays 3'7, 38, 22 and 35 oftrunkcircuit T2, relay 66 of emergency alarm apparatus EATL and relay 164 oftrunk circuit. RD2 all release. Should the operator disconnectbeforethecalling subscriber releases the connection, relay 3'7 releasesbut relay 41 holds relay 24 and relay 24 holds relay 22. When thecalling subscriber releases the connection, relays 41, 24 and 22 releaseand the switches S1 and LF are restored to normal.

When a trouble condition arises in the branch oifice, an alarminitiating relay such as either of relays '74 or 84 of the alarm circuitAL is operated, the operation of relay '74 being effective to operaterelays '75 and 76 to initiate a class A alarm and the operation of relay84 being effective to'operate relay 85 to initiate a class B alarm. Theoperation of eitherof relays '76 or 85,..closes a. circuit from groundthrough a front contactof the operated relay, over conductor '79,through the left back contact of relay 83, winding ofrelay 80, backcontact and winding of magnet 90 of switch TS, to battery through themain power fuse 69. The operation of either of relays '76 or 85 alsoconnects the winding of magnet 90, through the back contact of relay80,. right back contact'of relay 83, front contact of relay 76 or 85,through brush 93, to the sleeve terminal of the'trunk with which thebrushes of switch TS are in contact. If this trunk is busy, the groundpotential connected over conductor 13 to its sleeve terminal in the bankof switch TS, as hereinbefore described, is effective to short-circuitthe winding of relay and cause the operation of stepping magnet 90. Withmagnet 90 operated, the energizing circuit through its Winding is openedto cause its release, each operation and release of magnet 90 beingeffective to advance the brushes of switch TS one step. Theswitch TS isadvanced step by step by'the operation of magnet 90 until brush 93encounters the sleeve terminal of an idle trunk, at which time relay 80operates in series with magnet 90. The magnet 90 is marginal and doesnot operate in series with relay 80. The-operation of relay 80 extendsthe ground from conductor '79 through its inner lower front contact tothe sleeve brush 93v as a guarding potential to prevent seizure of thistrunk by any of the selectors such as S1.

Assuming'the selected set of terminals to be connected to trunk circuitT2, relay 80 connects ground through .its upper front contact andthrough brush 94 over conductor 14 to cause the operation of relay 22 oftrunk circuit T2. The operation of relay 22 causes the operator at themain office to be signaled as hereinbefore described for a callinitiated at station A. When the operator answers the call, theoperation of relay 3'7 over the trunk 150 and the resulting operation ofrelay 38, both as hereinbefore described, complete a circuit foroperating relay 81 of the alarm circuit AL. This circuit is traced frombattery through the winding of re- :lay 81, lowermost front contact ofrelay 80, brush 95 of switch TS, over conductor 15, through theuppermost front contact of relay 22 of trunk circuit T2, and through theupper front contact of relay 3'7 to ground at the middle upper frontcontact of relay 38. Relay 81 closes a circuit for operating relay 82,and connects ground to conductor ,89, to start the ringing machine (notshown) Relays 24 and 25 of trunk circuit T2 are not operated and relay38'closes a circuit for operating relay 39, from battery through theupper winding of relay 39, lowermost front contact of relay 38, leftback contact of relay 25, to ground at the inner lower front contact ofrelay 38. Relay 39 looks through its innerupper front contact to thesame ground and also to the guarding ground connectedto sleeve conductor13 by brush 93 of switch TS. In the case of a class B alarm, relay 81also connects the right-hand winding of the dial tone transformer 8'7 inseries with condenser 86, through the inner left front contact ofrelay85, back contact of relay '76, and left front contact of relay 81to brush 91 of switch TS, thence over conductor 11 to trunk circuit T2,through the uppermost front contact of relay 38, left upper winding ofrepeating coil 40, uppermost front contact of relay 39, through theupper winding of relay 41 to ground. The tone circuit is ineffective tooperate relay 41 but is effective to inductively transmit throughrepeating coil 40 a class B trouble tone to the operator in the mainoffice. In case of a class A trouble, the secondary winding of troubletone transformer '78 in series with condenser 77 is connected throughthe outer left front contact of relay '76 and left front contact ofrelay 81, thence as traced to ground through the upper winding of relay41, thereby transmitting a class A alarm tone to the operator in themain office. The aforementioned operation of relay 82 of the alarmcircuit closes a circuit for holding relay 80 operated, and closes acircuit for operating relay 83. Relay 83 locksthrough its frontcontactover conductor '79 to ground at the front contact of relay 76 or backcontact of relay '75. The ground potential connection for guarding thetrunkv T2 is now traced from conductor '79, through the outerfrontcontact of relay 82 instead of the left back contact of relay 83, andthrough the inner front contact of relay 82 instead of the right backcontact of relay 83. When the main office operator disconnects, relay3'7 releases, but relay 38 holds temporarily through its inner upperfront contact in parallel with relay 22 over conductor 14 to ground atrelay 80 of alarm circuit AL. The release of relay 37 causes the releaseof relays 81, 82 and 80 of the alarm circuit in the order named, relay83 remaining operated as long as either of the alarm initiating relays"76 or 86 remain operated, that is until the trouble no longer exists.The release of relay 82, disconnects the busy potential from brush 93,thereby releasing relays 22 and 38 of trunk circuit T2. relays 39 and47, and relay 22 releases relay 35, the trunk circuit T2 being restoredto normal for use in regular trafiic.

7 Should the operator at the main office wish to check the existence ofa class A or class B trouble condition at the branch office, the trunkis seized as hereinbefore described, the number dialed by the operatorbeing a number assigned to the miscellaneous alarm circuit AL. When aselector such as T18 and a connector such as Relay 38 releases C havebeen selectively operated to-seize the terminals connected to the line70 of alarm circuit AL, a ringing circuit is closed from the source ofringing'current (not shown) in connector C, through condenser 71 and thewinding of relay 72. Relay 72 responds to alternating ringing currentand if either of relays 76 or is at that time in an operated conditionas a result of an existing trouble condition, ground is connectedthrough resistance 88 and the front contact of relay 72, to the ringconductor of line 70, thus operating the tripping relay (not shown) ofconnector 0. With relay 85 operated the class B tone source is connectedto the ring conductor of line 70 and with relay 76 operated the class Atone source is connected to the ring conductor of line 70, the tone ineither case being transmitted over the talking circuit through connectorC, switch-TIS, trunk circuit T2 and trunk 156 to the calling operator.If no trouble condition exists and neither of relays 85 or 76 isoperated, ground is not connected to the ring conductor of line 70, andringing is continued without being tripped, thus serving to indicate tothe calling operator that no trouble condition exists. 'When theoperator disconnects from the trunk, the trunk is released for use inregular traffic.

When an emergency arises in the branch ofiice, such as power failure,the normal signaling circuits are or may be ineffective to transmitsignaling current to the main ofiice. It being necessary that amaintenanceattendant be summoned immediately, one of the trunks to themain office is provided with emergency alarm apparatus EA1 at the branchofiice and with emergency alarm apparatus Ell-2 in the main omc'e. Thetrunk 150 is so equipped. Theapparatus EA-l comprises an independent andreliable source of signaling current 60 arranged for connection to thetrunk conductors leading to the main oifice when the emergency troublecondition arises. The closing of such a connection is efiective tooperate the signal apparatus EA2 at the main oilice to signal theoperator. A special tone source is included in the apparatus EA1 forconnection to the trunk to give the answering operator an emergeneyalarm tone. The source of signaling current 60 may be a 135-volt drybattery such batteries being of known reliabilityand long life fornoncontinuous and infrequent use. The apparatus EA-2 comprises the neonlamps 171 and 172 connected to the conductors 151 and 1.52 of trunk 156,an alarm relay 176, the alarm drop signal 177 and the alarm answeringjack 178. The relay 54 in the alarm circuit AL represents alarm relayswhich are operated in case of the operation of a relay rack fuse, theoperation of a switch frame fuse, in case of the failure of the ringingsupply, or in case the automatic battery charging.apparatusfails tooperate when required. With all of the alarm relays represented by relay54 normal, relay 55 of the emergency alarm apparatus EA1 remains normal.Relays 56 and 57 are normally operated being connected directly to themain power fuse 69. Relay 57 normally connects trunk conductors 51 and52 to trunk 150 for use in regular traffic as hereinbefore described.

When the main power fuse 69 blows or when relay 55 operates, due-totheoperation of one ofthe emergency alarm relays 54, relays 56 and 57release The release of relay 57 disconnects the conductors of trunk 150from trunk circuit of buzzer relay 63, and through'resistance 65 toground. The interruption of this circuit bycontacts' of relay 63 inducesa potential difference across right winding of this relay, thusconstituting, in combination with condenser 54 and the windings oftransformer 62, a

tone source from which a tone is transmitted through condenser 61 andthe back contacts of relay 5'7 over trunk 150 to the main'ofiice. Relay56 also connects ground through its upper back contact and the backcontact of relay 66 over conductor 53 and through the back contact ofrelay 24, over conductor 26 of trunk circuit T2 to sleeve conductor 13to guard this trunk circuit against seizure while the trunk 150 is beingused for the emergency alarm. Should the trunk- 150 be busy on anincoming or an outgoing call at the time the emergency alarm conditionarises, relay 66 will have been operated over conductor 18 to ground intrunk circuit T2, in which case the connection of ground to conductor53, by the release-of relay 56, is delayed; when the trunk circuit T2restores to normal due to release by the calling subscriber on anoutgoing call, or to the disconnection of trunk circuit T2 from trunk150 atrelay 57 on an incoming-call, relay 66 is released and theguarding ground potential at relay 56 is then connected to sleeveconductor 53.

The potential difference across the neon tubes 171 and 172 of alarmapparatus EA-2, when the source of signaling current 60 of emergencyalarm apparatus EA1 becomes connected to the conductors of trunk 150, issuiiicient to in jack 178, relay 176 is released, signal 177' isrestored, and the tone generated by the buzzer relay 63 in the branchoffice is received through condenser 173 and 174. When the emergencysignal apparatus EA-1 is restored to normal, the source of signalingcurrent 60 and tone transformer 62 are disconnected from trunk 150, thetrunk 150 being reconnected to trunk circuit T2 by the operation ofrelay 57. The busy potential is disconnected from sleeve conductor 13 oftrunk circuit T2 by the reoperation of relay 56.

The arrangement of apparatus EA-2 is such that the alarm relay 176 willnot respond to the connection of low potential or a ringing currentsource to the conductors of trunk 150.

The connection of a non-grounded high potential across the conductors oftrunk is also ineffective to operate the alarm since relay 176 would beoperatively unaifected even though which may arise during normal orabnormal operating conditions.

What is claimed is:

1. In Y combination, a two-conductor line, means at one end of said linefor impressing a potential on both of said conductors in parallel, andsignaling means at 'the other end of said line comprising two likedevices bridged across said. conductors, said devices havingapproximately infinite electrical resistance when subjected todifierences of potential below a certain critical value and having acomparatively low resistance when subjected to differences of potentialequal to or greater than said critical value.

2. In combination, a two-conductor line, two devices bridged across oneend of said line, each of said devices having approximately infiniteelectrical resistance to differences of potential below a certaincritical value and having a com-.

paratively low resistance to diiTerences of potential equal to orgreater than said critical value, a relay connected to the mid-point ofsaid devices, and means for connecting a source of current to both ofsaid conductors in parallel, said source being of high enough potentialto cause an electrical current through said devices and relay, andsignaling means controlled by said relay.

3. In combination, a two-conductor line, two space discharge devicesconnected in series across one end of said line, signaling meansconnected to the mid-point of the connection between said devices, andmeans at the other end of said line for connecting a source of currentto both of said conductors in parallel, said sourcev being of highenough potential to cause the operation of said devices. and saidsignaling means.

4. In combination, a two-conductor line, means at one end of said linefor impressing a low electrical potential difierence across saidvconductors, means at the same end of said line for impressing a highelectrical potential on both of said conductors in parallel, two spacedischarge devices connected in series across the other end of saidconductors, and signaling means connected to the mid-point of theconnection between said devices, said devices and signaling means beingoperatively responsive to the impression of said high potentialandoperatively non-responsive to the impression of said low potential.

5. In combination, a two-conductor line, means at one end of said linefor impressing a difference in electrical potential across saidconductors, means for impressing an electrical potential on both of saidconductors in parallel, two space discharge devices connected in seriesacross the other end of said conductors, and

signaling means connected to the mid-point of the connection betweensaid devices, said devices and signaling means being operativelyresponsive to the impression of an electrical potential to both of saidconductors in parallel and operatively non-responsive to the impressionof a difference in potential across said conductors.

6. In combination, a two-conductor line, a source of ringing current andmeans for connecting said ringing current source across said conductors,means for impressing an electrical potential on both of said conductorsin parallel, and signaling means at the other end of said linecomprising two space discharge devices connected in series across saidconductors and a relay, said means being operatively responsive to theimpression of an electrical potential on both of said conductors inparallel and operatively non-responsive to the connection of a source ofringing current across; said conductors.

7. In a telephone system, a first ofiice, a second office, subscriberslines in each oihce, a group of trunks for use in completing callsbetween said offices, alarm apparatus located in one of said offices forindicating a trouble indication in said other ofiice, said apparatusbeing associated with one of said trunks and comprising two spacedischarge devices connected in series across the conductors of saidtrunk and a relay, and means in said other office responsive to theoccurrence of a trouble condition for conmeeting. an electricalpotential to both conductors of said trunk, said, potential being highenough to break down the resistance of said devices and operate saidrelay. I

'8. In a telephone system, an unattended office, an attended ofiice,subscribers lines in each of said ofiices, a trunk extending betweensaid offices for completing calls, a source of ringing current at theunattended oifice for connection across the conductors of said trunk tosignal an operator at the attended omce, a high potential source ofcurrent at said unattended oiiice having one of its terminals connectedto ground, means operative in case of an emergency for connecting theother terminal of said high po-' tential source to both of theconductors of said trunk, and alarm equipment at said attended oiiiceoperatively responsive to the connection of said high potential butunresponsive to ringing current from said unattended office.

9. In a telephone system, a first ofiice, a second office, a groupoftrunks for use in completing calls between said offices, means in thefirst office operative in response to a trouble condi- 1 tion forselecting an idle one of said trunks,.

means for transmitting a trouble indicating signal over the selectedtrunk to the second oflice, a central office source of potential for useon calls initiated or terminating in said first office, an-

ble condition in the first office for connecting said other source ofpotential to said trunk, and means at said second ofiice responsive tothe connection of said other source of potential to said trunk forsignaling an operator in said second office.

10. In a telephone system, a first office, a second ofiice, a group oftrunks for use in completing calls between said offices, means in thefirst office operative in response to a trouble condition for selectingan idle one of said trunks, means for transmitting a trouble indicatingsignal over the selected trunk to the second office, a central officesource of potential for use on calls initiated or terminating in saidfirst oflice,

another source of potential for use in transof said other source ofpotential to the conductors of said trunk.

11. In a telephone system, a first ofiice, a second oifice, a trunkextending between said offices for use in completing calls, apparatus insaid first oifice associated with said trunk for use on calls incomingand on calls outgoing over said trunk, a central office source ofpotential for use on calls originating or terminating in said firstoffice, another source of potential for use in transmitting an emergencyalarm signal over said trunk, and means operatively responsive to anemergency trouble condition in said first oifice for disconnecting saidtrunk from said apparatus.

12. In a telephone system a first office, a sec-- ond oifice, a trunkextending between said offices for use in completing calls, apparatus insaid first ofiice associated with said trunk for use on calls incomingand on calls outgoing over said trunk, a central ofilce source ofpotential for use on calls originating or terminating in said firstoifice, another source of potential for use in transmitting an emergencyalarm signal over said trunk, and means operatively responsive to anemergency trouble condition in said first office for disconnecting saidtrunk from said apparatus and for connecting said other source ofpotential to said trunk.

13. In a telephone system, a first oifice, a second office, a trunkextending between said offices for use in completing calls, apparatus insaid first ofiice associated with said trunk for use on calls incomingand on calls outgoing over said trunk, a central office source ofpotential for se on calls originating or terminating in said firstoifice, another source of potential for use in transmitting an emergencyalarm signal over said trunk, and means operatively responsive to anemergency trouble condition in said first ofiice for disconnecting saidtrunk from said apparatus and for connecting said other source ofpotential to both conductors of said trunk in parallel.

14. In a telephone system, a first office, a second office, a trunkextending between said offices for use in completing calls, apparatus insaid first office associated with said trunk for use on calls incomingand on calls outgoing over said trunk, a central office source ofpotential for use on calls originating or terminating in said firstoffice, another source of potential for use in transmitting an emergencyalarm signal over said trunk, means operatively responsive to anemergency trouble condition in said first ofilce for disconnecting saidtrunk from said apparatus and for connecting said other source ofpotential to said trunk, and signal means at said second ofiiceoperatively responsive to the connection of said other source ofpotential to said trunk.

15.111 a telephone system, a first office, a second ofilce, a trunkextending between said offices for use in completing calls, apparatus insaid first office associated with said trunk for use on calls incomingand on calls outgoing over said trunk, means operatively responsive to atrouble condition other than an emergency trouble condition for seizingsaid trunk, apparatus in said second ofiice associated with said trunkfor use on calls incoming to and outgoing from said second office andfor use in responding to an alarm signal other than an emergency alarmsignal received over said trunk, other apparatus associated with saidtrunk in said second office for use in receiving an emergency alarmsignal from said first ofiice, a central office source of potential foruse on calls originating or terminating in said first ofiice,

another source of potential in said first office,

and means operatively responsive to an emergency trouble condition insaid first ofiice for connecting said other source of potential to saidtrunk.

16. In a telephone system, a first office, a second ofiice, subscribersstations and lines, a trunk connecting said offices, a central ofiicesource of potential for use on calls originating or terminating at asubscribers station in said first ofilce, another source of potential insaid first office, apparatus in said second ofiice associated with saidtrunk for use on calls incoming to and outgoing from said second ofiiceand for use in responding to an alarm signal other than an emergencyalarm signal, emergency alarm apparatus normally connected to said trunkin said second office, an operators link in said second ofiice forconnection with a trunk, means responsive to the connection of said linkwith said trunk for disconnecting from said trunk the emergency alarmapparatus in said second oifice, and means in said first officeoperatively responsive to an emergency trouble condition for connectingsaid other source of potential to said trunk for operating the emergencyalarm apparatus in said second oflice.

17. In a telephone system, a first ofiice, a second ofiice, subscribersstations and lines, a trunk connecting said offices, a central ofiicesource of potential for use on calls originating or terminating at asubscribers station in said first office, another source of potential insaid first ofiice, apparatus in said second office associated with saidtrunk for use on calls incoming to and outgoing from said second office,emergency alarm apparatus normally connected to said trunk in saidsecond office, an operators link in said second ofiice for connectingwith said trunk, means responsive to the connection of said link withsaid trunk for disconnecting from said trunk the emergency alarmapparatus in said second ofiice, means in said first office operativelyresponsive to an emergency trouble condition for connecting said othersource of potential to said trunk for operating the emergency alarmapparatus in said second ofiice, and means for transmitting an emergencytrouble tone over said trunk during the connection of said other sourceof potential to said trunk.

CLOSMAN P. STOCKER.

